Most color picture tubes presently being manufactured are of the line screen-slit mask type. These tubes have spherically contoured faceplates with line screens of cathodoluminescent materials thereon and somewhat spherically contoured slit-apertured shadow masks adjacent to the screens. A screen 10 and a shadow mask 12 of an early type of line screen-slit mask tube are shown in FIGS. 1 and 2, respectively. In this type, the screen 10 is formed with curved sides 14, rounded corners 16 and straight vertical lines 18. The shadow mask 12 includes slit apertures 20 arranged in straight vertical columns 22. The screen is formed utilizing a photographic technique that uses a line light source for exposure and the shadow mask as a photographic master. Because of the generally spherical shape of the shadow mask 12, the off-axis slit apertures do not parallel the line light source during screening. This nonparallelism results in the formation of jagged phosphor lines on the screen. Such jagged lines are undesirable.
One technique used to overcome this problem of jagged screen lines is illustrated in FIGS. 3 and 4. A screen 24 is formed with bowed lines 26 as shown in FIG. 3. Such bowing is concave toward the vertical axis Y--Y, with the curvature of the screen lines increasing with increasing distance from the vertical axis Y--Y. In the corresponding shadow mask 28, the aperture columns 30 are similarly bowed concavely toward the vertical axis Y--Y, as shown in FIG. 4. Because of this bowing of the aperture columns 30, the longitudinal axes of the slit apertures 32 that are off the major axis X--X and off the minor axis Y--Y are closer to parallelism with the line light source during screening. Thus the bowed lines 26 are smoother than are the lines of the embodiment of FIG. 1. Patents illustrative of this bowed screen line and bowed aperture column concept are: U.S. Pat. No. 3,889,145, issued to Suzuki et al. on June 10, 1975; U.S. Pat. No. 3,925,700, issued to Saito on Dec. 9, 1975; and U.S. Pat. No. 3,947,718, issued to van Lent on Mar. 30, 1976.
Recently, several color picture tube modifications have been suggested. One of these modifications is to decrease the curvatures of the faceplate and shadow mask and to square-off the viewing screen, so that the peripheral borders have straight sides and the corners are substantially square. In another modification, the curvatures of the faceplate and shadow mask remain unchanged, but the peripheral border of the screen is changed to square-off the corners of the screen.
In these modified tubes, it is desirable to obtain substantially straight lines in all portions of the screen, and it is particularly desirable to form straight lines at the sides of the screen. In some of the abovementioned tube modifications, however, it is impossible to obtain straight lines at the screen sides by utilizing conventional construction designs and techniques. The present invention provides a solution to this problem by utilizing an improved novel pattern of slit columns in the shadow masks of such tubes.